This is the Q9450 and Q9400 head to head. The relationship for the Q9550 and Q9505 should be similar. http://www.anandtech.com/bench/default.aspx?p=51&p2=76&c=1
Pretty similar for productivity stuff. Gaming actually uses the extra L2 effectively, but might not be worth the extra money.
You can change the CPUs at the top, but the Q9505 isn't listed but see what the E8xxx are too to see if the apps you'll be using will benefit.